Photomasks are used in a wide variety of applications including the fabrication of semiconductor integrated circuits such as ICs, LSIs and VLSIs. Basically, the photomask is prepared from a photomask blank having a chromium based light-shielding film on a transparent substrate, by forming a predetermined pattern in the light-shielding film by photolithography using UV or electron beams. The current demand for a higher level of integration in the semiconductor integrated circuit market has created a need for a smaller pattern rule. The traditional solution is by reducing the wavelength of exposure light.
However, reducing the wavelength of exposure light improves resolution at the sacrifice of focal depth. This lowers the process stability and adversely affects the manufacture yield of products. One effective pattern transfer method for solving the problem is a phase shift method. A phase shift mask is used as a mask for transferring a micro-pattern.
Referring to FIGS. 9A and 9B, a phase shift mask, specifically a halftone phase shift mask is illustrated as comprising a substrate 1 and a phase shifter film 2′ deposited thereon. The mask consists of a phase shifter 2a that forms a pattern on the substrate and an uncovered area 1a of the substrate 1 that is exposed where the phase shifter 2a is absent. A phase difference of about 180° is set between light transmitted by the uncovered substrate area 1a and light transmitted by the phase shifter 2a. Due to light interference at the pattern boundary, the light intensity at the interfering boundary becomes zero, improving the contrast of a transferred image. The phase shift method permits to increase the focal depth for acquiring the desired resolution. This achieves improvements in resolution and exposure process margin, as compared with conventional masks having ordinary light-shielding patterns in the form of chromium film.
Depending on the light transmission of phase shifter, the phase shift masks are generally divided for practical application into full transmission type phase shift masks and halftone type phase shift masks. The full transmission type phase shift masks are transparent to the exposure light wavelength because the light transmittance of the phase shifter section is equal to the light transmittance of uncovered substrate areas. In the halftone type phase shift masks, the light transmittance of the phase shifter section is several percents to several tens of percents of the light transmittance of uncovered substrate areas.
FIGS. 10 and 11 illustrate the basic structure of a halftone type phase shift mask blank and a halftone type phase shift mask, respectively. The halftone type phase shift mask blank shown in FIG. 10 has a halftone phase shift film 2′ formed over substantially the entire surface of a substrate 1. Patterning the phase shift film 2′ results in the halftone type phase shift mask which is shown in FIG. 11 as comprising phase shifter sections 2a forming the pattern on the substrate 1 and uncovered areas 1a of the substrate where the phase shifter is absent. Light that passes the phase shifter section 2a is phase shifted relative to light that passes the uncovered substrate area 1a. The transmittance of the phase shifter section 2a is set to a light intensity that is insensitive to the resist on a wafer or article subject to pattern transfer. Accordingly, the phase shifter section 2a has a light-shielding function of substantially shielding exposure light.
The halftone phase shift masks include single-layer halftone phase shift masks featuring a simple structure and ease of manufacture. Some single-layer halftone phase shift masks known in the art have a phase shifter of MoSi base materials such as MoSiO and MoSiON as described in JP-A 7-140635.
As mentioned above, the halftone phase shift mask is an effective means for accomplishing a high resolution in a simple manner. For accomplishing a higher resolution, there exists a requirement to reduce the exposure wavelength from the current mainstream wavelength of 248 nm (KrF laser wavelength) to a shorter wavelength of 193 nm (ArF laser wavelength) or even 157 nm (F2 laser wavelength). In order for a halftone phase shift mask to accommodate light exposure at such shorter wavelength, one common approach is to reduce the compositional ratio of metal (e.g., Mo) to silicon of which the halftone phase shift film is made.
To deposit a halftone phase shift film containing metal and silicon in such a lower metal/silicon ratio, a metal silicide target having a low metal content is generally sputtered in an atmosphere containing such gases as oxygen and nitrogen. Several problems arise with the metal silicide target having a low metal content.
The metal silicide target used in depositing a halftone phase shift film is typically a sintered target. In the sintering step involved in its manufacture process, the sintering temperature must be lowered because the metal silicide target having a lower metal/silicon ratio has a lower melting point. This fails to produce a target having a satisfactory sintered density. If a target having an unsatisfactory sintered density is used in the deposition of a halftone phase shift film, there can arise a problem that particles are released from the target during sputter deposition, resulting in increased film defects.
The method contemplated effective for avoiding such an increase of film defects by the particles from the target is a co-sputtering process in which a high density target consisting of metal and silicon in a high metal/silicon ratio (e.g., MoSi2 target) is combined with a metal target or silicon target (e.g., single crystal silicon target). Both the targets used in this process have a sufficiently high density, and film defects due to the particles release from the target are suppressed to some extent.
In order for a halftone phase shift film to accommodate the above-discussed reduction of wavelength for light exposure, however, the halftone phase shift film containing metal and silicon must have a lower metal/silicon ratio than the prior art halftone phase shift films, typically less than 0.15 in molar ratio. In depositing a halftone phase shift film containing metal and silicon in such a lower metal/silicon ratio, the co-sputtering process encounters a problem that the sputtering power applied across the target having a high metal content (e.g., MoSi2 target) largely differs from the sputtering power applied across the target having a low metal content (e.g., single crystal silicon target), that is, there is a significant difference in power density (W/cm2) between these targets. Since it is impossible to apply powers to both the targets at optimum power densities, the composition within the halftone phase shift film being deposited becomes inconsistent, leading to large variations of transmittance. The electric discharge of sputtering also becomes unstable, sometimes resulting in even increased film defects.